Our goals for the coming year are: 1. Determine the biochemical basis of the mutator we discovered in Salmonella typhimurium. We are well on the way toward purifying the mutant polymerase and it shows abnormal properties. 2. Improve our in vitro test for mutators and antimutators. 3. Study the effect of external agents on the fidelity of DNA replication. 4. Apply our tests to animal systems.